The invention relates to infrared radiation detector devices and in particular to pyroelectric infrared radiation detector devices.
The use of pyroelectric material for infrared radiation detection is well established. Pyroelectric material is employed in detector devices by making a body of the material into a capacitor with electrodes on oppositely located plane surfaces perpendicular to the direction of polarization of the material. When the material suffers a temperature change, for example, as a result of infrared radiation incident thereon, the electrical polarization in the material changes. The redistribution of compensating charges on the surface of the material produced thereby causes a current to flow in a circuit in which the capacitor is connected.
Pyroelectric infrared radiation detector devices are used in a variety of applications. Their characteristics make them particularly attractive for application in remote switching systems, intruder detection systems and in movement sensing generally. In such applications, the detector device responds to a moving object by detecting the flux change produced in passing through the device's field of view. In the case of the object being a person, the infrared radiation emitted by the moving person or body part is converted by the detection device into an electronic signal which can be used to actuate an alarm or to switch lights on and off.
Various pyroelectric materials have been employed for such purposes. These materials include triglycine sulphate, modified lead zirconate titanate, lithium tantalate and certain plastic film materials such as polyvinylidene fluoride. Polarization is normally achieved in such materials by applying an electric field in the direction of the polar axis, sometimes while simultaneously subjecting the material to an elevated temperature, so as to align the electric dipoles. In some materials, for example, L-alanine doped triglycine sulphate, it is not necessary to induce polarization as this exists already.
It is possible to use in, for example, an intruder detection system, a detector device comprising a single detector element of pyroelectric material. However, for the purpose of detecting small movements across the total field of view of the detector device an increased sensitivity can be obtained when the device comprises two, or more, detector elements. Advantageously, such an arrangement can be used to provide some immunity from undesired signals caused by variations in ambient temperature and background radiation by connecting the two pyroelectric elements differentially. In one configuration, the device is constructed so that uniform changes in input radiation in the fields of view of both elements, for example changes in background radiation, will produce voltages across the pair of elements which are in opposition and therefore no net signal voltage is created, whereas a change of input radiation in the field of view of just one element can produce a differential output signal.
One such known radiation detector device comprises a pair of detector elements arranged in a common plane and each formed of a body or body part of polarized pyroelectric material. The elements each have electrodes on opposite major surfaces in overlapping relationship, and the electrodes extend generally normal to the polarization direction. The elements within the device are electrically connected via their electrodes to form two series connected capacitor detectors in which the directions of polarization of the pyroelectric material are in opposition. Such detector devices are commonly referred to as "Dual" detector devices.
In an alternative known form of Dual detector device, the two detector elements may instead be connected in parallel, but still with opposite polarities. This arrangement has the advantage that it usually provides a better signal to noise ratio than the aforementioned series-opposed arrangement. As in the series-opposed arrangement a common single body of pyroelectric material may be used.
Dual detector devices are highly suitable for use in intruder detection systems. One important advantage is that fluctuations in the thermal state of the background produce no output signal from the device. In single element detector devices, such fluctuations produce a noise-like signal which can result in false alarms.
To ensure a well focussed image, which is highly desirable for a dual detector device, and an acceptable operating range, the infrared radiation is usually focussed by an optical system. This can be in the form of a molded plastic mirror having a number of facets with coincident foci. The mirror being constructed such that the detector device can be mounted at the focus without obscuring the field of view. As an intruder, for example, moves across the field of view of the mirror, a corresponding number of separate images will be focussed onto each element of the device individually, ensuring multiple triggering signals. Alternatively a Fresnel lens array may be used.
The detector devices normally include an impedance converting amplifier, typically a n-channel field effect transistor (FET) whose gate is connected to, for example, the series connected detector elements, and a nonlinear network which protects the gate of the FET from excessive voltages and progressively limits the pyroelectric voltage resulting from large changes in ambient temperature so as to prevent overloading.
While these dual detector devices perform reasonably well, (with the two detector elements serving to reduce false triggering instances resulting from changes in ambient temperature, background temperature and acoustic noise), they are not wholly immune to false triggering. Thermal and electronic noise within the pyroelectric elements and their associated circuitry can still give rise to false triggering. Random noise spikes can occur from time to time which are of a magnitude sufficient to produce an output signal from the detector device and its associated circuitry.
Attempts have been made to alleviate this problem. Thus, in an intruder detection system for example, it is known to provide an alarm generating circuit which is designed to respond to at least two output signals from the detector device within a given time period. Since false triggering due to random noise spikes occurs fairly infrequently, the likelihood of two such triggering signals occuring within the given time period is remote. However, the necessity for the system to produce at least two triggering signals before generating an alarm can, in some circumstances, impair the effectiveness of the system in detecting intruders and for this reason the system is not considered to be entirely satisfactory.
In another known arrangement, a so-called two channel detector, the detector device effectively comprises two dual detectors each having two, spaced, pyroelectric detector elements comprising pyroelectric material and electrodes on opposite surfaces thereof. The two pyroelectric detector elements of each dual detector are connected in a circuit differentially so as to obtain therefrom an output dependent on temperature differences between the two detector elements.
In the known arrangement, the four pyroelectric elements of the two dual detectors are spaced from one another in a linear array, with the first and third elements of the array being connected together to form one dual detector, and the second and fourth elements being similarly connected together to form the second dual detector. The outputs from the two dual detectors are monitored, and an alarm signal is generated in response to signals provided by both detectors.
A signal from one of the detectors by itself as a consequence, for example, of a noise spike rather than an intruder will not result in an alarm output. The chance of noise spikes occuring simultaneously in both dual detectors and causing an alarm output is small.
While this arrangement might seem therefore a satisfactory solution to the problem of false triggering, it is not without disadvantages. In use, the image of an intruder will traverse the elements from one side to the other of the array. Thus the image will move across onto the first element of the first dual detector, producing as a result a signal, and then across the gap between that element and the first element of the second dual detector, this being the next in the series array, before falling on this second detector element to produce a second signal.
The time taken by the image to traverse the elements of both dual detectors in this manner can cause difficulties. As the signals from both dual detectors will not be coincident, it is necessary to initiate a timing period or window, typically in the order of several seconds, following the occurrence of a signal from one of the dual detectors in which a signal from the second dual detector is expected to be received. If a signal from the second dual detector occurs during that predefined time period, the system responds accordingly to provide an alarm. If, on the other hand, a signal is not received from the second dual detector during that time period, the system assumes the signal from the first dual detector to be a spurious signal, caused for example by a noise spike, and resets itself, thus effectively ignoring that first signal.
This timing period can lead to difficulties in operation. The timing period must be of sufficient duration to allow for the image of the intruder to pass from a detector element of the first dual detector to a detector element of the second dual detector. If the timing period selected is too short, there is risk that the delay between the times the image of an intruder falls on respective adjacent elements of the dual detectors would be greater than the timing period, with the result that the intruder is not detected by the system. If the timing period is unduly long, there is a very real risk of false operation of the system since non-intruder related noise spikes could still occur in both channels during that timing period leading to the false triggering. It will be seen therefore that the detecting efficiency of this system could, in certain circumstances, be impaired.